The present invention relates to a primer composition, and in particular, to a unique primer composition for an aluminum substrate. The primer composition has excellent adhesion to an aluminum substrate and has outstanding resistance to elevated temperature and high humidity conditions.
Epoxy adhesives have been widely used in the aircraft industry to bond together aluminum parts. However, failures of these bonds have occurred within hours under conditions of high stress and elevated temperature and high humidity, for example 50.degree. C. and 95% R.H. and a shear stress equal to about 30% of the original ultimate strength of the bond. To reduce these adhesive bond failures, the aluminum has in the past been coated with a corrosion inhibiting primer containing hexavalent chromium. Chromates provide the best adhesion heretofore available by inhibiting surface metal corrosion by retarding the anodic electrochemical reaction of aluminum with water. Said primers offer superior durability and resistance to hostile environments over primers using other anodic corrosion inhibitors such as permanganates and silicates.
While chromate corrosion inhibitors perform quite well, they are both toxic to man and the environment. The novel primer composition of this invention contains no toxic materials and yet has mechanical performance similar to that of chromate based primers. It is believed to derive its surprising effectiveness from the hydrophobic nature of silicone polymers and from the inherent strength of aluminum-phosphate bonds. The novel primer composition of this invention has excellent adhesion to aluminum substrates and provides a surface to which the epoxy adhesive will adhere and the primer does not fail when subjected to stress and elevated temperature and high humidity conditions. Combining these desirable features with a formulation which is not toxic to man and the environment represents a substantial improvement over prior art formulations.